Optical fibres whose refractive index profiles show the above-mentioned modulation structures are to be understood to mean in this connection in particular multimode fibres having a length dependence of the transmission band width corresponding approximately to the square root of the fibre length, as well as single mode fibres having polarization-maintaining properties over large fibre lengths.
When multimode fibres are used, large transmission bandwidths can be realized when the refractive index profiles thereof show an ideal previously calculable variation which is constant over the whole fibre length in particular with a view to the rotation symmetry. These requirements can only be satisfied now with any of the known manufacturing methods in a restricted way. Dependent on the process, systematic radial profile errors (dip, layer structure, etc.), rotation asymmetries and axial fluctuations occur which in practice reduce the transmission bandwidth of such graded index multimode fibres typically to less than a tenth of the theoretically possible values.
The influence of such profile errors on the transmission behaviour, however, can be reduced in that periodic modulations in radial and/or axial directions with exactly defined modulation amplitudes and modulation frequencies are impressed upon the refractive index profile. These modulations may simultaneously result in the transmission bandwidth no longer decreasing linearly but only in accordance with the square root of the fibre length (Appl. Opt. 20 (1981) 2314-2318; DE-AS No. 27 33 872).
Such a behaviour is desirable in particular in the case of transmissions over large track lengths, as will become obvious from the following example: when graded index multimode fibres are used with a bandwidth-length product of 1.2 GHz.km the overall transmission bandwidth in a system having a length of 18 km in the case of a linear dispersion characteristic, is only approximately 65 MHz. In the case of a dispersion with a square root of the fibre length dependency, however it is approximately 280 MHz. Fibres having ideal, rotation symmetrical and length-constant refractive index profiles show however an unfavourable, that is, linear, pulse broadening due to intrinsic transmission properties. Therefore impression of specific profile modulations is desirable also when the refractive index profiles are ideal. This would consequently result in an improvement of the system-transmission properties.
In the deposition according to the MCVD-, OVD and VAD methods, profile fluctuations are generally produced for reasons which are inherent to the method. These profile fluctuations influence the conductivity properties partly unfavourably, for example, in case of profile approximations with a low number of layers and partly also favourably in the desired direction, for example, in the case of helical profile modulations in the VAD process (Appl. Opt. 20 (1981) 2314-2318). The methods mentioned hereinbefore, however, have in common that the modulations automatically occur and cannot be adapted via a specific variation of deposition parameters to the theoretically required optimum modulation structures. The reasons are inherent to these methods. For example it is necessary to fuse the soot particles produced homogenously in the gaseous phase. This principally leads to large deposition areas, to a high thermal inertia, and to the necessity of a continuous rotation of the substrate tube during the deposition. These disadvantages do not occur in the deposition according to the PCVD method (see for example, U.S. Pat. Re No. 30 635 and U.S. Pat. No. 4,314,833). In particular a local delay-free deposition at low substrate temperatures of glass layers is possible even without rotating the substrate tube and without using a separate sintering step. It is known that with this method refractive index profiles with high rotational symmetry and substantially ideal radial profile variations can be manufactured, in which profile fluctuations are substantially avoided (EP-OS No. 132 011).
For certain fields of application, such as optical sensors and wide band telecommunication transmission, it is desirable to manufacture monomode fibres having profile structures which maintain the polarization condition of the transmitted light. In such profile structures the core geometry or the refractive index profile shows a marked elliptical, in general a non-rotational, form over the whole fibre length, which theoretically should lead to a suppression of the polarization dispersion otherwise occurring. The methods known so far for the manufacture of such structures via elasto optic asymmetries or subsequent change of the geometry of the deposited material (for example, in accordance with EP-OS 67 017), however, are very time-consuming and hence expensive or are only effective in a restricted way.